1. Field of the Invention
This invention relates to the art of blow molding of thermoplastic materials. More particularly, it relates to ways and means for blow molding hollow shaped articles from a normally highly crystalline thermoplastic material such as polypropylene and the like, that must be biaxially oriented to achieve certain desirable properties such as high strength, clarity, and the like.
2. Description of the Prior Art
The manufacture of shaped hollow articles, for example, containers such as bottles and the like, from said thermoplastic material has been accomplished by blow molding methods. Two such general methods start with a continuous tube that has been extruded from the thermoplastic material, cooled to crystallize the material, and then subjected to heat treatment. Examples of such heat treatment are disclosed in the U.S. Pat. Nos. Re 26,956 and 3,496,258, to Wiley, and the U.S. Pat. No. 3,677,530, to MacDuff.
In one method a piece of the tube, the material of which generally is at its orientation temperature, is cut and the cut piece, referred to as a prison, is grabbed at each end and longitudinally stretched. While the parison is in the stretched condition, the mold pieces of a blow mold are closed upon the tube, and the tube is expanded, generally by blowing air into the parison, to form the body of the desired article. The mold pieces are then opened, the formed body is removed, and the cycle is repeated. This method has a number of disadvantages, one of which is that its production rate is inherently low.
In the second general blow molding method, the tube is basically left intact. In one embodiment of this general method a "hand-over-hand" longitudinally stretching and blow molding technique is used to make a chain of hollow bodies of the desired shaped article. This is shown in the U.S. Pat. Nos. 3,699,199, 3,704,188 and 3,798,295, to MacDuff, and the U.S. Pat. Nos. 3,751,542 and 3,834,852, to Hall. In this embodiment two movable blow molds are involved. The mold pieces of the first blow mold at a first position are closed on the tube, and the blow mold is then moved away a predetermined distance to a second position. During this movement of the mold the portion of the tube within the blow mold is blown into the shape of the mold cavity, and the portion of the tube trailing the blow mold is pulled by the blow mold from a first point to a second point. Throughout the pull of the tube or at a preselected time during the pulling of the tube, its travel past the first point is restricted as by a restrictor cone or by a clamp so that the portion thereof between the two points becomes longitudinally stretched by the time the first mold reaches the second position. In the meantime, the second blow mold with its mold pieces open and straddling the first blow mold with its mold pieces closed is returned from the second position to the first position, and the mold pieces are closed on said stretched portion of the tube. The mold pieces of the first blow mold are opened. The second closed blow mold then is moved to the second position to push away the blow molded body from between the opened pieces of the first blow mold, and at the same time pull the trailing portion of the tube from the first point to the second point. Throughout the pull of the tube by the second blow mold or at a preselected time during the pull, the movement of the tube past the first point is restricted, and the portion of the tube between the two points becomes longitudinally stretched by the time the second blow mold has reached the second position. As in the case of the first blow mold, during this forward movement of the second blow mold the stretched portion of the tube within the mold is blown to expand the tube into conformance with the configuration of the mold cavity, whereby the body is formed and the material becomes biaxially oriented. The blow molded body pushed from between the opened pieces of the first blow mold is linked by an unexpanded portion of the tube to a previously formed chain of linked together blow molded bodies. This chain is conveyed from the machine to a finishing operation in which the bodies are separated from the links and trimmed, and the article mouths or necks or other openihgs are bored or cut to give the finished articles.
The production rate of the "hand-over-hand" machine and method is limited to a large extent by the number of molds in the machine. To increase the production rate, therefore, more molds should be added. In constructing new machines this can be done readily, if there are no space limitations or monetary constraints. In such instance the first blow mold becomes two blow molds which are moved and for the most part operated together, the second blow mold becomes two blow molds which likewise are moved and for the most part operated together, whereby there is symmetry in structure and operation, and the machine is lengthened so that the extent of longitudinal stretch of the tube is substantially the same each time it is stretched.
Where, however, an existing machine or a new machine with space limitations or monetary constraints is involved, the addition of two molds might not be feasible, whereas the addition of one blow mold might be. Hence, a problem to which this invention provides a solution is how to incorporate one additional blow mold into a "hand-over-hand" two-mold machine and method, and yet have the longitudinal stretch of each length of the tube on which the mold pieces of each blow mold close substantially the same.
The machine and method described herein are sufficient improvements of the "hand-over-hand" machine and method.